Shell Programming and Scripting

_lock_dir=<some_lock_dir>
_lock_file=${0##*/}.lck

[ -f "$_lock_dir/$_lock_file" ] && [ ! -L "$_lock_dir/$_lock_file" ] && {
  printf >&2 'another instance of %s is already running: $s\n' "${0##*/}" "$(< "$_lock_dir/$_lock_file")"
  exit 1
  }

[ -d "$_lock_dir" ] || 
  mkdir "$_lock_dir" || {
    printf >&2 'failed to create lock dir: %s\n' "$_lock_dir"
    exit 1
    }

printf > "$_lock_dir/$_lock_file" '%d\n' $$

<the rest of your code here ... >

# remove the lock file at the end

[ -f "$_lock_dir/$_lock_file" ] && [ ! -L "$_lock_dir/$_lock_file" ] &&
  rm -- "$_lock_dir/$_lock_file" || {
    printf >&2 'failed to remove lock file: %s\n' "$_lock_dir/$_lock_file"
    exit 1
    }